I attributed it to not having FP+ but maybe it's a combo of that and having the meet-and-greet in a park that both sees lower attendance overall and isn't (yet, at least) thought of for most people, as a place to take young children for this kind of thing.
... Of course, high wait times in the MK may have also been the result of a dearth of frozen related attractions in that park to handle consumer demand for the IP. What's more interesting than a meet-and-greet? A ride, so the wait may be lower as a result of what people perceive as a better use of their waiting time next-door, too.
It's probably a combo of all of the above but it feels more satisfying to pin it strictly on FP+ so I'm going to stick with that, I think!